The present invention relates to a method of filling and sealing a gas capsule, particularly a gas capsule of a kind that holds a discrete volume of pressurised gas at high pressure, say, between 30 bar and 80 bar.
Sealed gas capsules are well known for use in circumstances where the force of the gas, typically carbon dioxide, under pressure is employed to dispense a substance such as draught beer from a beer dispenser or soda water from a soda siphon. Such capsules have a body portion and a neck portion, the latter typically being provided with a seal which is able to be broken so as to release the gas.
It is also known to employ such sealed gas capsules containing helium at high pressures in medical devices using the energy of the pressurised helium to drive a therapeutic agent through the skin of a patient. The storage of helium presents particular problems in view of its high fugacity.
In PCT published application WO94/24263 there is described a needle-less syringe, which includes a metal capsule containing helium gas at high pressure which is used to force particles of a therapeutic agent through the skin of a patient in a substantially painless manner. The capsule is detachable from the remainder of the syringe and once used, either a new charge of gas can be placed in the capsule or more favourably the capsule can be discarded and a new capsule charged with gas can be attached to the remainder of the syringe.
In the circumstance where the gas capsule is a “throw away” item it is important that it can be manufactured simply and cheaply. In medical applications helium gas is a favoured fluid since it is very light which makes it suitable for use as a propellant for therapeutic agents in that when it impinges against the skin of a patient it will bounce off into the atmosphere and not pass through the skin of the patient. However, helium because it is light, is difficult to contain since it will leak through the most minuscule fault in a container.
U.S. Pat. No. 4,727,233 relates to the sealing of metal tubes, particularly of stainless steel, for example, tubes that are used to fill containers of gas at high pressure. In the method according to U.S. Pat. No. 4,727,233, once a predetermined quantity of gas has been supplied to the container, the latter is sealed by the cutting and sealing of the tube through which it is filled. The following procedure is adopted. A pair of main jaws which carry so-called presser nibs are operated to squash part a chosen portion of the tube. A pair of auxiliary jaws which also carry presser nibs are detachably engaged to the main jaws. Accordingly, operation of the main jaws causes the auxiliary jaws to squash the rest of the chosen portion of the tube. The squashing of the chosen portion of the tube is carried out in such a manner so as to achieve a completely sealed constriction of the tube. If desired, a bush may be pre-inserted in the chosen portion of the tube so as to facilitate the formation of this seal. The auxiliary jaws are then disengaged from the main jaws and taken away so as to free part of the squashed part of the tube, while the remaining part of the squashed portion remains under the clamping pressure exerted by the main jaws. The tube is then severed, for example, by means of cutting blades, at the freed part of the squashed portion to form an end having lips. The lips are then welded, for example, by plasma-arc welding.
The procedure set out in U.S. Pat. No. 4,727,233 has a number of disadvantages. First, the need to disengage the auxiliary jaws from the main jaws and then to sever the tube adds considerably to the length of time required to perform the entire sealing operation and therefore makes it unsuitable for use in industrial production when the tubes are required on a continuous basis. Second, the cutting of the tube may create small particles of metal that may interfere with the subsequent welding process or even become trapped in the lips making possible their subsequent release under pressure if the sealed gas container is to be opened by puncturing the weld. Such release of metal particles could be injurious to a patient if the gas is employed for the transcutaneous delivery of a pharmaceutical.